1. Field of the Invention
The present invention relates to a solid-state image sensor and an imaging system including it.
2. Description of the Related Art
Solid-state image sensors such as a CMOS image sensor have been studied to decrease the number of elements (e.g., MOS transistors) which form a pixel, in order to downsize the pixel. As one method, Japanese Patent Laid-Open No. 2004-343529 proposes an arrangement in which a selection transistor for selecting a pixel is omitted. In this arrangement, a pixel is selected by controlling the voltage of a floating diffusion (to be referred to as an FD hereinafter) coupled to the gate of an amplifier MOS transistor. In the arrangement described in Japanese Patent Laid-Open No. 2004-343529, the FD voltage is reset in the readout period during which a signal is read out from each pixel to a column circuit arranged on each column. This readout period is a horizontal blanking period.
In the solid-state image sensor, the frame rate decreases as the readout period (horizontal blanking period) becomes longer. According to this pixel selection method, the FD voltage of pixels on a selected row needs to be set high, and that of pixels on an unselected row needs to be set low. Before reading out signals from pixels belonging to a selected row, a low voltage (selection disable voltage) needs to be applied to the FDs of all pixels. Then, a high voltage (selection enable voltage) needs to be applied to the FDs of pixels on a selected row.
A relatively long time is taken to set the voltage of an entire voltage supplying line to a selection disable voltage level. If this time is long, this means that the pixel selection disable period occupies a large proportion of the time taken to read out signals from all pixels. This problem is more serious in an application requested of high frame rate or a case where the imaging plane becomes large to increase the resistance or parasitic capacitance of a voltage supplying line. In particular, the HD (High Definition) standard requires an output of 60 frames per sec, and strong demand has arisen for solving this problem.